1. Field of the invention
The invention relates to a method for the production of effervescent granules, in which the reactants are reacted in a vacuum in an evacuatable container with evolution of gas.
2. Background of the Invention
Effervescent granules and effervescent tablets can be prepared by mixing of alkaline constituents eliminating carbon dioxide, in particular bicarbonates or carbonates, with preferably edible, organic acids and subsequent direct tabletting, by granulation of the effervescent components in a fluidized-bed drier or granulation and subsequent drying on trays. In the preparation of effervescent granules, however, granulation in a vacuum is becoming increasingly important.
Vacuum granulation is advantageous in the so-called “reactive” granulation for effervescent granules, in which the acids and/or the acidic salts thereof are reacted with the alkaline constituents and are granulated. The reactive constituents react with one another after addition of polar solvents, such as water or mixtures of water with alcohols or binder solutions or as a result of hydration of at least one reactive constituent. However, particular process control measures are required for controlling this reaction.
PCT/US94/02870 describes a method for the granulation of effervescent components, the granules being dried after the reaction in a vacuum. The granulation is effected at atmospheric pressure and with stepwise addition of the liquid, with the result that the reaction can be better controlled. By applying a vacuum and increasing the temperature, the reaction—before the complete conversion of the acid—between the effervescent components is stopped and the excess liquid is removed. However, the information is not specific and gives no indication of the determination of the end product of the reaction by measurement of the evolution of carbon-dioxide.
U.S. Pat. No. 4,824,664 describes a method in which the liquid required for the reaction is aspirated in a vacuum of 600 mbar. The reaction then takes place under atmospheric pressure over a period of from 25 to 40 minutes, whereupon the reaction is stopped by drying in a vacuum. On the one hand, this is much too slow for an efficient production method and, on the other hand, the reaction takes place in an uncontrollable manner in this time span.
U.S. Pat. No. 4,678,661 describes a method in which the reaction is allegedly controlled by measurement of the pressure difference which, however, is insufficient without controlling the time factor. After respective intermediate drying, raw materials are added, and the reaction is started again by adding liquid, which lengthens the production time in an undesired manner.
EP-B1-76 340 describes a granulation method for a pulverulent or granular mixture in a closed system in a vacuum, a pressure above the resulting partial pressure of the solvent and below atmospheric pressure being maintained. The vacuum at the start of the reaction must be as low as possible (from about 10 to 20 mbar). During the vacuum treatment, a metered amount of the solvent is added to the mixture for passivating the surface of at least one of the reaction components. After 1000 mbar has been reached by the evolution of carbon dioxide from the time of addition of the solvent, the mixture is dried. This treatment—addition of solvent and drying—is repeated until the surface passivation indicated by substantial slowing down of the reaction or reduced gas evolution is reached. The amount of carbon dioxide evolved at 1000 mbar serves as a parameter for the degree of passivation of the surface. The course of the reaction between 10 and 1000 mbar can easily lead to an overreaction and undesired considerable granulation owing to the water formed in the reaction and not removed by suction.
In U.S. Pat. No. 4,911,930 a hot air or vapor stream is aspirated into the granules by means of reduced pressure and cannot serve for controlling the reaction.
Disadvantages of the methods mentioned are that the parameters required for carrying out an optimum reaction are neither uniquely reproducible from batch to batch nor definable independently of the influence of the batch-related differences in raw material. The reaction of the effervescent component is also influenced by the water formed during the reaction. Depending on the quality of the raw materials, the reaction can take place more weakly or more vigorously, with the result that different amounts of water form per unit time. Owing to these varying reaction circumstances, the control of the method by the time alone or by the carbon dioxide measurement alone is decisively more difficult or the highly desired possibility of automation is virtually ruled out.
If only the time span (as, for example, according to U.S. Pat. No. 5,312,626 or EP-A1-525,388) in which the reaction takes place is regarded as an essential parameter, this may vary in the case of different raw material qualities, for example in the case of different residual moisture content, particle size, etc. of the acids and/or their acidic salts or the alkali metal constituents eliminating carbon dioxide and can lead to different results, such as to excessive granulation with agglomerate formation, or to insufficient granulation.
It has therefore been found that the methods corresponding to the prior art were not suitable for achieving standardized methods for fully automatic production. In addition—as already mentioned—the intermediate drying and the repetition of the granulation step lengthens the production times to an undesired extent.